Memory devices are used to store data. The memory devices are classified into volatile memory devices and nonvolatile memory devices. Data stored in volatile memory devices are disappeared when their power supplies are interrupted while data stored in nonvolatile memory devices are retained even when their power supplies are interrupted.
Since nonvolatile memory devices can store data using a low power, they are in the limelight as a storage medium of a portable device. There is a flash memory device as a kind of a nonvolatile memory device. In below, a flash memory device is described as an example of a nonvolatile memory device. However, a range of the present inventive concept is not limited to this but can be applied to other nonvolatile memory devices (e.g., a charge trap flash memory, PRAM, FRAM, MRAM or the like).
Data stored in a memory cell of a flash memory device (or a charge trap flash memory) is distinguished by measuring a threshold voltage of a memory cell. A threshold voltage of a memory cell is determined depending on the number of electrons stored in a floating gate. As the number of electrons stored in a floating gate increases, a threshold voltage becomes high.
Electrons stored in a floating gate may be leaked by many causes. Electrons stored in a floating gate may be leaked by an external stimulus (e.g., heat). Also, electrons stored in a floating gate may be leaked by wear of a memory cell. A repetition of an access operation in a flash memory device wears an insulating layer disposed between a channel region and a floating gate. The access operation includes a program operation, an erasing operation and a read operation. If an insulating layer is worn, charges stored in a floating gate are easily leaked. A decrease of a threshold voltage causes a read error, thereby degrading reliability of a flash memory device.